The present invention relates to a novel process for producing polycondensates, and to polycondensates thus obtained.
Sequenced block polymers and copolymers, referred to hereunder as "block (co)polymers" containing acrylic or methacrylic units for which we shall use the generic term "(meth)acrylic" below, are of great importance and are widely used in industry. The properties that arise due to the presence of such blocks make them suitable for numerous applications; nevertheless, such applications are sometimes limited due to the low molecular weight of such block (co)polymers. Moreover, a perfectly defined structure, such as a star arrangement with a determined number of branches or a particular sequencing, in other words a particular topochemical structure, would enable such block (co)polymers to be endowed with other properties. Many efforts have been made to increase the molecular weight of block co-polymers containing poly(meth)acrylate blocks, notably by coupling. Efforts have also been made to alter their sequencing to lead, for example, to tri-sequenced A--B--A polymers by coupling of bi-sequenced A--B polymers, these latter being difficult to obtain directly, or to polymers having a star structure. Attempts have thus been made to couple polymers A, or A--B, A--B--A, A--B--C etc. copolymers, in order to obtain block (co)polymers of type A-- A, A--B--B--A, A--B--A--A--B--A, A--B--C--C--B--A, etc. as well as star (co)polymers.
Coupling is a known technique for certain polymers such as for example, polystyrene and polybutadiene. In this case, dialdehydes are employed as coupling agents, following reaction between a carbanionic species and the carbonyl group. For other units, a great variety of techniques, coupling agents, and, in certain cases cross-linking agents, exist.
Numerous polymerisation techniques, notably for (meth)acrylic units, have been developed during recent years. Among these, group transfer polymerisation (GTP) should be mentioned. This technique leads to "living" polymers being obtained, in other words carrying an anionic termination which is hence highly reactive, enabling functionalised polymers to be prepared-which can be coupled using a diisocyanate, along with polymers of a defined structure. Nevertheless, this technique suffers from the disadvantage of requiring preliminary synthesis of silylketene acetal as an initiator, and GTP cannot be applied to synthesis of sequenced copolymers containing non-(meth)acrylic sequences.
S. D. Smith in Polymer Preprints, vol. 29, No. 2, 1988, LN 3776, pp. 48-49 describes the reaction between the carbanion termination of a methacrylic unit and an aldehyde group. This reaction between "living" methacrylic anions and benzaldehyde is highly selective and leads to O.sup.--, --OH terminations, in other words functionalised terminations.
French Patent 2 469 400 discloses star copolymers and the process for obtaining them. The "branches" or arms of the star are formed by copolymers containing at least one block of (meth)acrylic units and are linked to a core constituted by a multifunctional cross-linking agent. This agent can notably be selected from polyol poly(meth)acrylates. Unfortunately, the number of branches is not controlled, and the resulting product does not have the required properties.
French Patent Application serial number 90 1724 describes functionalised multisequenced polymers and their production. These polymers contain poly(meth)acrylate blocks and are functionalised at their terminations. There is no mention of coupling in this patent application. Thus, the process described therein, when carried out in order to obtain high molecular weights or a predetermined tri-sequencing by direct synthesis, leads to considerable increase in manufacturing costs.